212 FLORIDA GEOLOGICAL SURVEY THIRD ANNUAL REPORT.
manent streams or bodies or fresh water. Furthermore, the seasonal distribution of the rainfall should be such as to pretty well balance the evaporation, otherwise the lakes and streams would fluctuate too much.
GENERAL DISTRIBUTION OF PEAT.
Peat is most abundant in regions of immature topography, i. e., where processes of erosion and sedimentation have been at work a comparatively short time, for it is in such regions that bodies of still, clear water are most numerous. Lakes, ponds, swamps, bogs, etc., in whatever manner they may originate, are comparatively short-lived, geologically speaking, for they are almost sure sooner or later to be either filled with sediment washed down from adjacent slopes, or drained by the gradual deepening of the channels leading out of them, or both. Regions of mature topography have no lakes or ponds, and the valleys are typically steep and narrow, with swift streams in them, toward their heads, and broad and level lower down. The lower courses of the streams are usually sluggish enough, but they are apt to fluctuate too mvch or carry too much mud for the formation of peat
In the eastern ,United States there are two extensive regions characterized by immature topography, namely, the glaciated region and the coastal plain. The glaciated region, which lies mostly north of the latitude of New York City, is believed to have been covered many thousand years ago with a slowly moving ice-sheet, or aggregation of glaciers, which scooped out innumerable irregular depressions (determined largely by the location and character of the rocks and ridges) in the surface, deposited dams of rock and gravel across many of the valleys, and changed the face of the earth in other ways. After the ice retreated northward and the climate became warmer again these depressions quickly filled with water, forming the beautiful lakes for which some of the northern states are noted. The time elapsed since the glacial period-only an insignificant fraction of geological time-has been too short for stream erosion to have much effect on this region, especially as the soil left by the ice-sheet is very largely sand and gravel, which resists erosion pretty well.* Many of the smaller lakes of this region have long since become filled with excellent peat, and the process is still going on actively in others. Similar conditions are found in Europe, mostly north of latitude 450, and to a lesser extent in some other parts of the world.
*In this connection see H. F. Cleland, Science II. 32: 82-83. July 15, 19I0.